1. Field of the Invention
The present invention relates to a liquid crystal display unit, and in particular to a liquid display unit which can reduce its drive voltage.
2. Related Art
As to a projection optical system for a conventional reflection type liquid crystal panel, a system using a polarization beam splitter has been prosperously used, as disclosed in, for example, JP-A2000-310823. Further, the display system for a conventional liquid crystal panel has been normally designed with such a premise that an incident beam impinges upon the panel, substantially normal thereto. In other words, the liquid crystal panel exhibits it maximum performance if the incident beam impinges upon the panel, substantially normal thereto.
In an example of a liquid crystal display system for a reflection type panel, as disclosed in SID 90 Digest page 327, liquid crystal molecules are oriented, substantially normal to a panel substrate, and a direction in which the liquid crystal molecules are inclined upon application of a voltage and the polarizing direction of an incident beam exhibit such a relationship that both directions are shifted from each other by an angle of 45 deg. Further, a transmission type panel has such a premise that an incident light impinges upon the panel, substantially normal to the panel. As to a typical transmission type liquid crystal display system, there may be exemplified a twist nematic mode disclosed in Molecular Crystal And Liquid Crystal Letters 2 (1985), page 139.
There may be exemplified several systems having such a premise that light is impinged upon a panel, oblique thereto, as disclosed, for example, Proceedings Of S. P. I. E. 3634, page 80, or JP-A-9-189809. The incident angle to the panel disclosed in the above-mentioned Proceedings Of S. P. I. E. 3634, page 80. is about 20 to 30 deg. at most. In this liquid crystal system used therein, component parameters including a thickness of a liquid crystal layer, are optimized into a condition in which the incident angle is shifted on the basis of a display system having such a premise that an incident beam basically impinges upon the liquid crystal layer. Further, even though the incident angle to the panel is about 30 deg., the angle is changed under Snell's law when the beam enters into a glass medium from the atmospheric air, and accordingly, the incident angle of the beam to the liquid crystal layer becomes small, that is, about 19.5 deg.
Alternatively, in a system disclosed in the JP-A-9-189809, although the incident angle to the panel is about 60 deg., the optical path is changed by a hologram element so that the incident angle to a liquid crystal layer becomes relatively small. That is, in this system, the incident angle to the liquid crystal layer is at most about 10 deg because the emergent angle cannot be large due to the constraint to the F-value. For example, if the incident angle to the liquid crystal layer is 10 deg., the emergent angle from the panel becomes 15 deg. which corresponds to about 2 as a F value. Further, the liquid crystal display system has such a premise that the incident beam impinges upon the liquid crystal layer as mentioned above, basically normal thereto.
In order to lower the cost of a liquid crystal panel used in a projection type liquid crystal display unit, it is most effective to reduce the size of the panel. In order to make the panel small-sized, it is required to decrease the size of pixels. The drive voltage of a liquid crystal display system using conventional nematic liquid crystal is general in a range from 3 to 6 Vrms which is not always said as low. Further, the liquid crystal panel requires a.c. drive, and accordingly, a drive voltage range from 6 to 12 V is required. Transistors constituting a control circuit for controlling the above-mentioned voltage, and transistors at the pixels should be of high voltage-resistant type. Thus, it is self-explanatory that there is a limitation to the miniaturization of the transistors, which, in turn, impedes the miniaturization of the pixels. Thus, in the conventional system, the size of the pixels is limited to a value about 8 to 10 μm.